OK guys. Thanks for your replies, but let's try and keep it simple and generic, because there's a lot of esoteric stuff here which I can't get my head around.
My scenario is this:
I planted my seedlings (autos) 12 days ago in 4 inch pots with good quality compost (don't ask for details!) and they're doing fine -2nd set of leaves sprouting nicely. Currently they are feeding sufficiently off the nutrients in my growing medium plus some pH. neutral water daily,
My question is - at what point should I be supplementing them with added nutrition? In other words, when will they run out of indigenous nutrients? And what would they look like at this stage?